Rubber network toughening

Rubber toughening is often assumed to be identical to the modification by mbber particles. However, there is an alternative and also very effective possibility of mbber toughening of amorphous polymers— the rubber network toughening (2). Here, small thermoplastic particles, eg, PVC particles (about 1 urn in diameter). 

The alternative and also very effective approach to rubber toughening uses rubber networks and is known as rubber network toughening [2]. Under uniaxial tensile load, the following deformation processes occur (see Fig. 5.15) ... 

As in the case of rubber particles it was demonstrated that the fracture energy is roughly proportional to the volume fraction of TP rich phase, both in the case of epoxy/PEI networks (Bucknall and Gilbert, 1989) and bismaleimide networks toughened with various TPs (Stenzenberger et al., 1988). This improvement was evidenced in the range of volume fractions below [Pg.415]

Section 5.2.6, Rubber-toughened PVC as disperse and rubber network system Section 5.3, Disperse systems with semicrystalline matrix ... 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

J. C. Hedrick, N. M. Patel, and J. E. McGrath, Toughening of Epoxy Resin Networks with Functionalized Engineering Thermoplastics, in Rubber Toughened Plastics, K. Riew (Ed.), American Chemical Society, Washington, DC, 1993. 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

Recent systematic studies on the relation between network structure and substituents in kraft lignin, steam exploded, have shown that the lignin containing networks can be modified in new ways, cf. e.g. (80). Also the toughening of glassy, structural thermosets can be achieved by incorporating a variety of polyether and rubber-type soft segment components in the polymer network structure. 

Swelling experiments showed that a lignin epoxide resin of 0.11 epoxy equivalents per lOOg formed a network polymer when cured with DETA, PA, or ATBN. Phase separation was observed in the rubber-toughened lignin epoxide network. Cured epoxides had lignin derivative contents of up to 95%. 

Figure 13.6 Relative contributions (%) of the different toughening mechanisms in epoxy networks versus temperature ( ) rubber bridging ( ) shear yielding and (A) cavitation. (From the results of Huang et a ., 1993b.)... 

The major limitation of rubber toughening of thermosets results from the fact that the increase in toughness can be achieved only at the expense of high-temperature performance or of mechanical properties, e.g., a decrease in modulus and yield stress. This can be unacceptable for structural and long-term applications (see Fig. 13.7). A second limitation is the lack of significant success in the toughening of high-Tg networks (see Fig. 13.8). 

Rubber plus Fillers - Toughened Networks (Hybrid Composites)... 

This chapter on applications of PAB s focuses on polymer systems giving synergistic and generally high performance properties. Low performance PAB s of commodity plastics, rubber toughened plastics, copolymers, and interpenetrating networks are excluded. Some of the more common PAB s are described elsewhere in this book. 

Hence a low molecular weight, reactive elastomer is normally used for impact modification of thermosets. The low molecular weight of the mbbery prepolymer aids its easy dissolution or dispersability in the thermosetting resin. The reactive functionality couples the rubber covalentiy to the growing polymer network during the curing reaction. Hence the rubber toughened thermosets may also be considered as co-reacted thermosets and not true blends. 

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